Practical optical devices must be fabricated so as to direct the light energy. Commonly, this is achieved by creating a waveguide. In the waveguide, a cladding layer of lower refractive index (typically 1.44) directs light by internal reflectance to an optical core of higher refractive index (typically 1.45–1.5). Both the core and cladding layer can be made from many different materials. Common materials include glasses of SiO2—GeO2, SiO2—B2O3—P2O5, SiO2—GeO2—B2O3—P2O5, SiO2 and SiON. Silicon dioxide, silicon nitride and silicon oxynitride are materials which are particularly valued for their optical properties, in particular their high optical transparency and wide range of refractive indices (1.45–2.5). These materials are used in a host of optical devices. The devices include, for example, planar waveguides, arrayed waveguides (AWG), wavelength demultiplexers, power splitters, optical couplers, phasers, and variable optical attenuators (VOA).
Typically, chemical vapor deposition (CVD) is used to deposit layers of silicon dioxide, silicon nitride or silicon oxynitride. In the CVD process, the substrate is placed on a heated susceptor in a quartz reaction chamber and then the reactant gases are introduced into the chamber. Typically, the gasses react on the surface of the substrate and form a deposited layer. However, some reactions may also occur as the gasses flow into the chamber. The most common gasses for the deposition of silicon dioxide, silicon nitride and silicon oxynitride are silane (SiH4), chlorinated silane (SiHxCl4-x), nitrous oxide (N2O), ammonia (NH3) and nitrogen (N2). These gases are inexpensive and can be purchased in great abundance.
Although the CVD process is the preferred process for depositing many of the materials used to manufacture optical devices, it is not without problems. The use of ammonia and silane in the production of silicon nitride and silicon oxynitride results in the incorporation of large amounts of hydrogen (up to 20 at % for silicon nitride) in the optical film.
The incorporated hydrogen generates significant optical losses at the 1550 nm optical communication band due to a strong overtone of the N—H bond. FIG. 1 illustrates the loss spectrum of conventionally processed silicon oxynitride, i.e., the loss over a range of wavelengths. The peak in loss is due to N—H absorption. In conventional manufacturing processes, the silicon oxynitride contains a significant amount of hydrogen. The figure clearly illustrates the deleterious effect of the overtone of the N—H bond. The center of the loss peak occurs at a wavelength of approximately 1510 nm. This is just 40 nm from 1550 nm, a preferred optical communications wavelength.
It is possible to remove much of the entrapped hydrogen with high temperature thermal annealing. However, the optical SiON film can blister and crack at the high temperature, rendering the device useless.
Therefore, it would be desirable to develop a method to manufacture optical devices which did not result in the incorporation of hydrogen in the optical SiON film and high losses at 1550 nm. Furthermore, it would be desirable to develop a process having the benefits of the speed and control of the conventional CVD process without resorting to a high temperature anneal to drive out the hydrogen.